The invention relates to a method for controlling an automatic transmission of a motor vehicle during spontaneous release of the accelerator pedal.
Modern automatic transmissions in motor vehicles are controlled by an electronic transmission control (EGS), which for automatic selection of a stored shift program which describes, for ex., a driver-type detection known from the publication xe2x80x9cATZ Automobiltechnische Zeitschriftxe2x80x9d 94 (1992), an environment detection, a driving situation detection or the detection of a manual engagement and for gear selection according to situations, communicates with other control units and computers of different aggregates.
By means of shift characteristic lines stored in the electronic transmission control, the re are established the operating points at which the electronic transmission control issues to the hydraulic system a command to upshift or downshift. Here the shift characteristic lines are usually presented as function of a vehicle speed and of a throttle valve position wherein the throttle valve position reproduces the position of the accelerator pedal, in old designs the throttle valve being mechanically connected with the accelerator pedal and electronically in modern designs.
When the driver takes his foot off the accelerator pedal, the cross-section of the opening of the throttle valve for gas release, as known already, diminishes, an upshift to an economy gear such as from the third to the fourth gear being introduced at sufficiently high speed of the vehicle.
However, this proves problematic in certain driving situations such as thrust operation, since the engine rotational speed drops with the upshift. This is a hindrance when acceleration is required or desired, since actually at least one downshiftxe2x80x94thereby taking up timexe2x80x94must again be carried out to be able to have available the desired engine power.
Also if such an upshift is carried out during a brief pedal release or gas release also called xe2x80x9cFastOffxe2x80x9d this can result, in a critical driving situation such as in an interrupted overtaking operation, not only in reducing the driving comfort but also in a dangerous situation. It is here desirable that the transmission remains in the actual gear at least for a certain amount of time, since firstly a delay of the vehicle is desired and secondly a rotational speed range with maximum input torque is favorable when, for ex., the overhaul operation is continued.
EP 0 574 965 A1 has disclosed a method for determining a shift signal from a shift characteristic field with which it is to be prevented that the transmission upshifts in the thrust operation and if that should be the case during transverse acceleration.
This is obtained by delaying the exceeding of upshift characteristic lines of the shift field by the operating point resulting form the momentary vehicle speed and a throttle valve position. According to this known method the position of the accelerator pedal is measured and when it goes beyond the upshift characteristic line it is converted to an accelerator pedal speed. The measured value thus obtained of the accelerator pedal speed is compared with a limit value stored in the transmission control. When said limit value is fallen below, a signal for upshifting is issued. The accelerator pedal speed is thus a criterion as to whether the transmission be upshifted or remains in the former gear. It is further proposed that the upshift operation be suppressed if needed when exceeding a transverse acceleration limit value.
However, in practice this method in which an upshift prevention is triggered according to a negative gradient of the accelerator pedal angle has provide disadvantageous in the sense that the upshift prevention, specially in brief pedal releases, is insufficiently sensitive. Thus the upshift prevention particularly often sets in too early or too late, or for too long or too short the driving is at a rotational speed too high for the actual operating situation. This in turn causes noticeable reduction of the driving comfort.
DE 41 20 566 has disclosed an other method for transmission control. In this known method, for activation of the upshift prevention system, in addition to the pedal angle gradient, the criterion xe2x80x9cthrust operationxe2x80x9d must be detected over a traction-thrust characteristic line, but this is not enough to ensure a correct shifting behavior during spontaneous gas release.
This invention is based on the problem of making available a method for control of an automatic transmission of a motor vehicle during spontaneous gas/pedal release with which an upshift prevention adapted to the driving situation is carried out so that during spontaneous pedal release a driving speed delay occurs and at the same time the greatest possible spontaneity is given for a subsequent acceleration.
In the inventive method the state of upshift prevention is advantageously activated not only according to a pedal position gradient but also taking into account the sportiveness of the driving behavior by the driver-type evaluation counter and the detection of the thrust operation according to engine rotational speed and road gradient. This makes possible a precise transmission adjustment with a shift strategy that does justice to the situation by being adapted to the environment, for ex., uphill or downhill driving, and to the driver type.
To the precision of the inventive method during a spontaneous gas/pedal release contributes that the state of upshift prevention is abandoned only when several criteria essential therfoe are satisfied. One of the these exit criteria consists is that the electronic transmission control must clearly detect traction operation. In addition limit values of the pedal position gradient and time limits according to the road gradient are tested as added exit criteria.
The use of characteristic fields in the inventive method has in addition clear advantages, specially relative to the use of limit values. While the problem exists when using limit values that at low pedal position value, that is, accelerator pedal only slightly depressed, there is only available a small resetting way and thus an accordingly lower pedal position gradient is given which is often below a limit value needed to activate the upshift prevention system, in a characteristic field where the curve of the pedal position gradient begins at zero, there exists the possibility of an upshift prevention during spontaneous pedal release even when the pedal is only slightly depressed.
While the state of upshift prevention is abandoned only after a first time, when a specific positive pedal position gradient has not been exceeded and in the case that the positive pedal position gradient be exceeded and again fallen below, a second time is started after which the state of upshift prevention is abandoned, when the positive pedal position gradient threshold has not again been exceeded during this time, the exit must only be introduced when a constant accelerator pedal position is detected with certainty.
With the inventive upshift prevention system during a spontaneous gas/pedal release increased comfort and greater security result in critical situations such as an interrupted overhaul operation.
In an advantageous embodiment of the inventive method it can be provided that a third time be started when a negative pedal position gradient is fallen below before the first time period has elapsed.
The exit from the state of shift prevention takes place here when traction operation is detected but a negative pedal position gradient threshold has been fallen below. Then a third delay time is started which is limited according to the road gradient, that is, uphill or downhill drive. Therewith it obtained that a repeated negative pedal position gradient be taken into account and the exit can be delayed.